[5.x] Ability to register custom image presets

[duncanmcclean]

This pull request makes it possible for addons to register custom image manipulation presets, which can be generated via php please assets:generate-presets.

Addons can register presets like this:

Facades\Image::registerCustomManipulationPresets([
    'og_image' => ['w' => 1146, 'h' => 600],
    'twitter_image' => ['w' => 1200, 'h' => 600],
]);

Related: statamic/seo-pro#258